1. Field of the Invention
The present invention relates to a rewritable photochromic optical disk having a photochromic recording layer.
2. Description of the Related Art
The photochromic optical disk making use of the isomerization reaction with light and heat which is reversible as in the organic photochromic materials, is known as one of the rewritable memories capable of recording, reading, and erasing such as an Erasable Direct Read After Writing (EDRAW) optical disk.
Such an erasable optical disk is produced by the following process. As shown in FIG. 1, a recording layer 2 including the organic photochromic material is formed as a thin film on a transparent substrate 1 and then an aluminum-made reflecting layer 3 is formed on the recording layer 2 in the form of a thin-film. A protective layer 4 is formed on the reflecting layer 3. On the main surface of the substrate 1, grooves used for tracking, prepits for controlling the writing and reading of data, and pre-addresses 5 are formed in advance by the process of stamping or the like. A laser beam 6 is irradiated on the the recording layer 2 of photochromic material through the substrate 1 of the disk.
The organic photochromic materials are compounds which react with light to repeatedly change from a chromatic state to an achromatic state or vice versa. As shown in light-absorption spectral distribution curves of FIG. 2 by way of example, the materials have the following characteristics. When light of a predetermined wavelength B is absorbed by the organic photochromic material, the material changes from a stable state X (first state) to a quasi-stable state Y (second state) in which it is rendered chromatic (chromatic reaction). When it absorbs light of another predetermined wavelength A or heat under the quasi-stable state, it returns to the original material again and is render achromatic (achromatic reaction). By utilizing the above described phenomenon i.e., photochromism, a rewritable optical disk is manufactured, to and from which information is recorded, reproduced, or erased by a laser beam. As such organic photochromic materials, there are for example, those materials such as thioindigo which make use of the isomerization reaction with light, spiropiranes which utilize the ring opening/closure reaction and a redox reaction or the like, and flugido, etc. In addition, other organic photochromic materials of diaryl ethene derivatives are disclosed in U.S. Pat. No. 4,837,063.
Recording of information on the optical disk is performed by applying energy of light and/or heat to the recording layer 2 by means of a writing laser beam in the form of spots through an optical pickup system to generate the chromatic reactions positively in the spots, thereby forming a train of chromatic spots in turn. The reading of information from the optical disk is performed by irradiating a reading laser beam having another wavelength so as to read out the train of the spots which have been recorded. The erasing of information from the optical disk is performed by irradiating a erasing laser beam having another wavelength to the train of the spots which have been recorded.
On the other hand, there is desired a rewritable photochromic optical disks capable of being recorded an played back information therefrom even by using a low power laser beam during the repeated writing, reading and erasing operations. However such optical disks do not been developed yet.